1. Field of the Invention
The present invention relates to a non-aqueous secondary cell containing an electrolytic solution which comprises an organic solvent. More particularly, the present invention relates to a non-aqueous secondary cell which has a small retention, that is, a difference between a charge capacity and a discharge capacity, a large discharge capacity, and safety against fire, etc.
2. Description of the Related Art
A non-aqueous secondary cell, a typical example of which is a lithium secondary cell, has a large discharge capacity and a high energy density and generates a high voltage. Then, its development is highly expected.
Usually, the non-aqueous secondary cell uses an electrolytic solution comprising an organic solvent and a lithium salt dissolved therein, and metal lithium or a lithium alloy as a negative electrode active material. When such negative electrode active material is used, the cell tends to suffer from the formation of an internal short circuit since the metal lithium induces the growth of dendrite or the hard alloy penetrates a separator during charging of the cell, whereby the cell properties are deteriorated, or the safety of the cell is impaired.
To overcome such problems, it has been proposed to use a carbon material such as activated carbon or graphite to or from which lithium ions can be doped or dedoped, as a negative electrode active material instead of the metal lithium or it alloy (see Japanese Patent Publication Nos. 24831/1992 and 17669/1993).
However, the known carbon material has a large retention since the electrolytic solution is decomposed on a surface of the carbon material. To obtain a cell having a large capacity, the decrease of the retention is effective.
In particular, in the case of the recent secondary cell, it is highly desired for the cell to have a high unit volume energy density of 200 wh/l or larger. With such cell, since it is required to make the best use of the active material of the cell, a secondary cell having a smaller retention is desired.
As the organic solvent in the electrolytic solution of the cell, propylene carbonate or 1,2-dimethoxyethane is used. A safer solvent is still sought.